The present invention relates to radiometers and, more particularly, to radio frequency radiometers.
In applications, such as satellite meteorology, requiring high resolution imaging of distant scenes, antenna bulk and power requirements are at a premium. This is especially true where radio frequency imaging is used, for example, to provide continuous observation despite cloud coverage. Many such applications have requirements exceeding the available operational capabilities of active imaging systems, e.g. radar.
For these reasons, a practical passive or radiometric imaging system is desired which operates at radio frequencies. Preferably, such a system would employ electronic scanning, since electronic scanning is not as power demanding as mechanical scanning.
Electronic scanning may involve phase gradient or time delay scanning, but these approaches are problematic in that the narrow bandwidths employed limit the power available for imaging to a small fraction of that actually arriving at the antenna. Broader bandwidth phase gradient and delay scanning systems encounter problems, such as those induced by a phenomenon known as frequency scanning, which causes the directionality of an antenna to shift as a function of frequency.
Some radar systems employ frequency scanning to advantage. For example, a travelling wave array is used as a frequency scanning line source transmit/receive antenna. A set of these arrays mounted side-by-side produces a scanning planar array. The planar array produces a scanning pencil beam, providing for a pixel-by-pixel reconstruction of the reflectivity distribution of a scene. A microwave transmitter changes the antenna beam direction by changing the input frequency.
In the frequency scanning radar, the receiver frequency tracks the transmitter frequency. The transmit and receiver beam have a common boresight. Target reflections produced by the transmit beam are therefore received with full sensitivity by the receive beam.
It would be desirable to use frequency scanning as an electronic scanning approach in passive systems. However, the control over operational frequency is lost, as is the predetermined common boresight, when target emission rather than reflected transmissions are being received. Furthermore, the emitters from radiometric scenes are noncoherent. For these reasons and others, heretofore, practical frequency scanning radiometers have not been developed.